In general, a linearity of an output signal from a power amplifier for radio transmission is limited, and gain reduction (linearity distortion) occurs especially when a level of an input signal is high. As a circuit for compensating the above-described linearity distortion, a Cartesian Feedback distortion compensating apparatus has been known. If the Cartesian Feedback distortion compensating apparatus operates ideally, the output signal of the power amplifier may have a high linearity.
In the Cartesian Feedback distortion compensating apparatus, the output signal of the power amplifier is extracted and then fed back to the input side. At this time, a phase change of a feedback system is produced due to influence by, for example, an antenna load, a transmission delay of a directional coupler or a demodulator, or the like. Therefore, to operate the Cartesian Feedback distortion compensating apparatus effectively, the phase change of the feedback system is required to be compensated.
To solve the above-described problem, Non-Patent Documents 1 (Joel L. Dawson, Thomas H. Lee, “Automatic Phase Alignment for a Fully Integrated CMOS Cartesian Feedback Power Amplifier System ISSCC2003”) and 2 (Joel L. Dawson, Thomas H. Lee, “Automatic Phase Alignment for a Fully Integrated Cartesian Feedback Power Amplifier System”, IEEE JOURNAL OF SOLID-STATE CIRCUIT, Vol. 38, No. 12, December 2003) have disclosed a phase correcting apparatus that is applied to the Cartesian Feedback distortion compensating apparatus. FIG. 1 illustrates a main part of this phase correcting apparatus.
With reference to FIG. 1, description will be made below of the conventional phase correcting apparatus. The phase correcting apparatus illustrated in FIG. 1 includes a power amplifier 2, a directional coupler 3, a local oscillator 4, an orthogonal demodulator 5, a phase difference detector 6, a phase rotator 7, and an orthogonal modulator 8. In the phase correcting apparatus illustrated in FIG. 1, an output signal (an RF signal) of the power amplifier 2 is extracted (is fed back) by the directional coupler 3, and the orthogonal demodulator 5 generates a baseband signal (a complex signal of I′ and Q′). The phase difference detector 6 detects a phase difference between the baseband signal (the complex signal of I and Q) that is to be input to the phase rotator 7 and the baseband signal (the complex signal of I′ and Q′) that is to be output from the orthogonal demodulator 5, and then gives the phase difference as a target phase compensation amount to the phase rotator 7. The phase rotator 7 rotates the baseband signal (the complex signal of I and Q) only by the given target phase compensation amount.
In this case, the phase difference detector 6 detects the phase difference as follows. If the amplitude and the phase on an I-Q plane of the baseband signal (I, Q) are r and θ, respectively, and if the amplitude and the phase on the I-Q plane of the baseband signal (I′, Q′) are r′ and θ′, the phase difference detector 6 performs calculation using the following formula.Δθ=G(IQ′−QI′)=−Grr′ sin(θ−θ′)
The phase difference detector 6 integrates d θ/dt in the above-described formula to calculate θ (the target phase compensation amount). The phase rotator 7 performs phase rotation only on the θ calculated by the phase difference detector 6 with respect to the baseband signal (I, Q). In the phase difference detector 6, the θ is not necessary to be given to the phase rotator 7, and the trigonometric function (sin θ, cos θ) may be given. According to the above-described configuration, the phase correcting apparatus illustrated in FIG. 1 compensates a phase change in a feedback system.
The conventional phase correcting apparatus has been basically designed with an analog circuit. In this case, especially the phase difference detector that detects the target phase compensation amount requires a highly accurate circuit. However, design of the circuit corresponding to signals in a wide range is difficult.